AC current measurement in the electric power industry has been traditionally carried out using transformers; voltage transformers and current transformers. In high-voltage environments, for example, when the line voltage is or is greater than 33 kV such as, for example, 400 kV, electrical insulation is difficult at least in part due to the safety performance requirements of magnetic core based current sensors, which can carry thousands of amps in its primary and 1A/5A of normal current in its secondary. The current sensors can sustain large fault currents of the order of about 20 to 30 times the normal currents for a sufficiently short period to allow power system protection equipment to trip circuit breakers for the faulty line. Furthermore, significant insulation, in the form of oil, is needed for such current transformers. Consequently, current transformers for such environments are bulky and expensive. However, any moisture/small gas bubble accumulations in the current transformer can lead to a catastrophic failure such as, for example, an explosion.
It is well-known to use optical current sensors or optical transformers within such high-voltage environments. These products are based on the Faraday rotation effect within optical fibres or within a bulk optical material in which the polarisation of an optical signal is affected by the magnetic field associated with an electric current carried by a conductor. Changes in polarisation are detected by an optical receiver. However, such products suffer from the disadvantages that the Faraday rotation effect is, firstly, relatively weak and, secondly, that detecting changes in polarisation is relatively difficult since polarisation within optical materials varies significantly with environmental conditions. Other environmental conditions such as, for example, the vibrations caused by weather, also adversely affect the performance of such optical current sensors. Still further, to compensate for instability, relatively large optical current transformers are required.
UK patent GB 2400172 B discloses an optical AC current sensor that is based upon an electro-optic amplitude modulator having a modulation depth that has fixed relationship with the driving voltage. The driving voltage is derived from an AC current or voltage under measurement. In operation, optical power from an optical source is modulated by the driving voltage, the modulation depth has a fixed relationship with the driving voltage and the modulated optical signal is detected by the optical receiver. In preferred embodiments, the electro-optic amplitude modulator is insensitive to polarisation variations due to using a diffractive MEMS based variable optical attenuator. Most types of electro-optical amplitude modulators or variable optical attenuators, including diffractive MEMS based variable optical attenuators, require a DC bias voltage to be able to change optical attenuation in both positive and negative directions. One skilled in the art will appreciate that such an active arrangement consumes power by requiring a separate circuit to provide a biasing voltage.
It is an object of embodiments of the present invention to at least mitigate one or more problems of the prior art.